Solve Differential Eccee5
1. **State the problem:** Solve the differential equation $$c^2 (y')^2 = y^2 (y^{2n} - c^2)$$ with initial conditions $$y(a) = A$$ and $$y'(0) = 0$$.
2. **Rewrite the equation:** We have
$$c^2 (y')^2 = y^2 (y^{2n} - c^2)$$
which implies
$$y' = \pm \frac{y}{c} \sqrt{y^{2n} - c^2}$$.
3. **Separate variables:** Assuming $y > 0$ for simplicity,
$$\frac{dy}{dx} = \pm \frac{y}{c} \sqrt{y^{2n} - c^2}$$
so
$$\frac{dy}{y \sqrt{y^{2n} - c^2}} = \pm \frac{dx}{c}$$.
4. **Integrate both sides:**
$$\int \frac{dy}{y \sqrt{y^{2n} - c^2}} = \pm \frac{1}{c} \int dx = \pm \frac{x}{c} + K$$
where $K$ is the integration constant.
5. **Substitution for integral:** Let $z = y^n$, then $dz = n y^{n-1} dy$, so
$$dy = \frac{dz}{n y^{n-1}} = \frac{dz}{n z^{(n-1)/n}}$$.
Rewrite the integral:
$$\int \frac{dy}{y \sqrt{y^{2n} - c^2}} = \int \frac{1}{y \sqrt{z^2 - c^2}} dy = \int \frac{1}{y \sqrt{z^2 - c^2}} \cdot \frac{dz}{n z^{(n-1)/n}}$$
Since $y = z^{1/n}$, then $y = z^{1/n}$ and $y = z^{1/n}$, so
$$\frac{1}{y} = z^{-1/n}$$
Thus the integral becomes
$$\int \frac{z^{-1/n}}{\sqrt{z^2 - c^2}} \cdot \frac{dz}{n z^{(n-1)/n}} = \int \frac{1}{n \sqrt{z^2 - c^2}} dz = \frac{1}{n} \int \frac{dz}{\sqrt{z^2 - c^2}}$$.
6. **Integral evaluation:**
$$\int \frac{dz}{\sqrt{z^2 - c^2}} = \ln \left| z + \sqrt{z^2 - c^2} \right| + C$$
7. **Putting it all together:**
$$\frac{1}{n} \ln \left| y^n + \sqrt{y^{2n} - c^2} \right| = \pm \frac{x}{c} + K$$
8. **Solve for $y$:**
Exponentiate both sides:
$$\left| y^n + \sqrt{y^{2n} - c^2} \right| = e^{n(\pm \frac{x}{c} + K)} = Ce^{\pm \frac{n}{c} x}$$
where $C = e^{nK} > 0$.
9. **Apply initial condition $y(a) = A$:**
$$\left| A^n + \sqrt{A^{2n} - c^2} \right| = C e^{\pm \frac{n}{c} a}$$
so
$$C = \left| A^n + \sqrt{A^{2n} - c^2} \right| e^{\mp \frac{n}{c} a}$$
10. **Final implicit solution:**
$$\left| y^n + \sqrt{y^{2n} - c^2} \right| = \left| A^n + \sqrt{A^{2n} - c^2} \right| e^{\pm \frac{n}{c} (x - a)}$$
11. **Check $y'(0) = 0$ condition:**
From the original equation,
$$y'(0) = 0 \implies y^{2n}(0) = c^2$$
so
$$y(0) = c^{1/n}$$
which may impose restrictions on $A$, $a$, and the sign choice.
**Summary:** The implicit solution is
$$\left| y^n + \sqrt{y^{2n} - c^2} \right| = \left| A^n + \sqrt{A^{2n} - c^2} \right| e^{\pm \frac{n}{c} (x - a)}$$
with the initial conditions used to determine constants and sign.